Concern over the acid drainage and radionuclide dissolution problems associated with current uranium tailings disposal methods, as well as the lack of fissionable thorium recovery, has prompted investigation into improved methods of isolating uranium and thorium from their ores while leaving solid tailings more suitable for disposal.
Present commercial uranium extraction in most mills usually involves leaching of the ore with sulfuric acid, ion exchange (or solvent extraction) separation of the uranyl sulphate complex from the leach liquor, and precipitation of the uranium most often as yellowcake (80-85% U.sub.3 O.sub.8). Fresh tailings from such operations usually contain approximately 3% pyrite and 300 or more picocuries (Ra-226) per g, which is well above current or projected environmentally-acceptable amounts. Environmental problems associated with this practice include generation of H.sub.2 SO.sub.4 within the tailings (caused by slow oxidation of pyrite) with the resulting slow leach of Th-230, Ra-226 and Pb-210 and some other radionuclides into the run-off. Presently, uranium mills treat the small amount (.about.1-5%) of the Ra-226 solubilized during leaching with barium chloride to precipitate a radium-barium sulphate which is stored in a separate tailings area. This latter precipitate can be broken down by bacteria leading to the additional problem of barium in any seepage water from the disposal area.
With the ore grades in many areas decreasing, and with more stringent requirements for environmental protection with respect to radionuclides in force, the technology as presently practiced at many locations will require basic modifications in order to meet such constraints.
Some reports have appeared on tests with HCl or chloride leaching of uranium. R. A. Ewing et al (Battelle Mem. Inst. Columbus, Ohio, Progress Rpts. Oct. 15, 1952, through to Oct. 20, 1954) tested the dissolution of carnotite and certain other U ores in HCl-acidified ethanol, -methanol and -acetone, obtaining U extractions of above 90% with HCl-methanol. In a pilot plant, U extractions of 95-98% were recorded. The use of KClO.sub.3 as an oxidizing agent was mentioned. No mention was made of the fate of thorium or radium-226. A recent attempt to verify these results on conventional Canadian Elliot Lake ores was unsuccessful (Ritcey et al).
U.S. Pat. No. 2,894,804, July 14, 1959, C. W. Sawyer et al describes extraction of uranium and radium from some uranium ores by treating with ferric chloride solution at temperatures above 50.degree. C. and pH less than 4. The leach solutions were treated with CaCO.sub.3 (pH.about.5) to precipitate iron, with BaCO.sub.3 (pH.about.6) to precipitate uranium, and H.sub.2 SO.sub.4 to precipitate radium. HCl was not tested in the Examples, nor was the fate of thorium followed.
W. A. Meerson et al (Izv. Akad. Nauk. SSSR Metal, I, 42, Jan.-Feb. 1967) extracted thorium and the rare earths from monazite sands using HCl solution after treating the sands with sodium hydroxide.
Some tests have been reported on high temperature chlorination of uranium minerals followed by extraction and recovery of uranium or certain other metals. R. Lepage et al (Trans. I.M.M., C 82, C101-102, June 1973) passed CCl.sub.4 over samples of Elliot Lake uranium ore between 600.degree.-900.degree. C. with maximum subsequent extraction and recovery of U reported to be 96% at 800.degree. C. Using instead Cl.sub.2 the recoveries were 91% at 800.degree. C. and 78% at 600.degree. C. G. R. Lachance tested CCl.sub.4 as a chlorinating agent for Canadian Beaverlodge ore (Eldorado Mining & Ref. Ltd., Rpt. T60-24, 1960). Amounts of U, Fe, silica, etc. (but not Th or Ra) were determined in the volatilized portion, in the residue, and in the calcine wash. Extractions of U varied between 11 and 98% depending on temperature, time, and CCl.sub.4 - and N.sub.2 -flow rates.
R. Cable et al (Met. and Chem. Eng. XVIII (9), 460-2, 1918) mention the recovery of radium, believing that radium in a pitchblende ore was converted to a volatile chloride by chlorination at high temperature. On washing the condensed chlorides apparently about 93% of the initial radium in this particular ore was recovered by coprecipitation with barium sulfate.
I. Adamskii et al (Nukleonica V(11), 761-769, 1960) investigated the chlorination of certain carboniferous sandstone and granite ores as well as wash containing 0.11% U, with extraction and recovery of U varying between 88 and 95%. Thorium or radium behaviour was not explored. Chattanooga shale has been chlorinated at 600.degree.-1000.degree. C. but the effect on Th or Ra was not determined (H. B. Rhodes et al, U.S. Pat. No. 2,890,099, 1959, and F. Z. Pollara et al, U.S. A.E.C. Rpt. RMO-4015, 1960).
G. Jangg et al (Atompraxis 7, 332-336, 1961) studied reactions between uranium oxides and Cl.sub.2, C, HCl, CH.sub.4, CCl.sub.4 and COCl.sub.2 and showed that the formation of UCl.sub.4 was not favored in the absence of the specified reducing agent. With a 2.5 times excess of CCl.sub.4, or Cl.sub.2 -- plus --CH.sub.4, 90% of the U was distilled as chloride at 600.degree. C. Thorium and Ra were not studied.
Japanese workers (Japan Pat. No. 1964-3015, Feb. 17, 1965; T. Suzuki et al, Tokyo Kogyo Shikensho Hokoku 63, 51-62, and 75-81, Feb. 1968; S. Ino et al, Kogyo Kagako Zasshi 68, 2360-6, Dec. 1965) treated a low grade U ore with mixtures of Cl.sub.2, CO and CO.sub.2, vaporizing about 80% of the uranium as chloride. Most of the Cl.sub.2 reacted with gangue material producing a raw condensate containing only about 4% uranium chloride without fractional distillation.
O. M. Hilal et al (Ind. Eng. Chem. 53, (12), 997-8, 1961) chlorinated monazite sands containing rare earths in a tube furnace at 900.degree.-850.degree. C. containing powdered charcoal. By maintaining a thorium collection zone above 475.degree. C., ThCl.sub.4 was deposited free of more volatile chlorides of U, Fe and P which condensed below 475.degree. C. Rare earth chlorides remained with the calcine.
In studying analysis of ores and yellowcakes for rare earths, involving chlorination at 950.degree. C. and volatilization of impurities such as Th, Fe, etc., J. B. Zimmerman et al (Anal. Chem. 32, (2), 243,1960) found that careful dehydration of the charge was necessary to prevent the formation of non-volatile refractory oxychlorides of thorium.
Thus little has been found in the literature on the direct HCl-leaching of uranium ores, and the extraction and recovery of thorium and uranium has not been described in this context. Initial chlorination of uranium-bearing materials has been described usually at temperatures well above 600.degree. C. One reference found that radium chlorides had volatilized from a pitchblende ore but we have not been able to repeat this work on conventional uranium ores. No feasible integrated process for the concurrent recovery of thorium and radium as well as uranium, has evidently been reported.